In a typical dispersive spectrometer, a scene is imaged onto a slit so that light from a thin region of the scene passes through the slit. Light from the thin region of the scene is then collimated, passed through a dispersive element such as a prism or grating, and imaged onto a focal plane. The resulting image on the focal plane is a spread spectrum image of the thin region of the scene passed through the slit. Thus, the spectrum of each pixel of the image from the slit may be recorded by a focal plane array. Usually, the slit is scanned over the image of the scene to create what is commonly called a data cube in which the spectrum for each slit pixel in a two-dimensional scene is stored.
A dispersive spectrometer may be used to examine a light source, it may be used to examine the reflectance characteristics of an object illuminated by the light source, or it may be used to measure the absorption characteristics of material when illuminated by a light source, to name but a few uses.
Referring to FIG. 6, there is shown a conventional dispersive spectrometer, generally designated as 60. As shown, dispersive spectrometer 60 uses prism 66 to disperse light into its constituent wavelengths. Radiant energy from scene 61 is collected by primary lens 62 and focused onto slit 63. Light from a thin portion of scene 61 passes through slit 63 and is collimated by lens 64. Prism 66 disperses the collimated light into its constituent wavelengths, according to the refractive properties of prism 66. The prisms role can be replaced by a reflective or transmissive grating without changing the basic nature of the spectrometer.
As shown, the resulting dispersed light is imaged by focusing lens 67 onto focal plane 70. As also shown, focal plane array 68 is disposed at focal plane 70 to detect the dispersed light from prism 66. The signal generated by focal plane array 68 is read out, digitized, and stored as data in data acquisition system 69. The data may be accessed by computer 65 for further processing and display.
The dispersive spectrometer shown in FIG. 6, as well as other dispersive spectrometers, are disclosed in greater detail in U.S. Pat. No. 6,816,258, issued to Richard Hutchin, on Nov. 9, 2004. The content of the patent is incorporated herein by reference in its entirety.
A broad spectrum spectrometer system, generally designated as 71, is shown in FIG. 7. As shown, light 80 enters module 79 which disperses the light into its constituent wavelengths. Module 79 may include a prism for dispersing the light. The spectrally dispersed light, generally designated as 78, passes through slit 74 before reaching detector array 73. Detector array 73 includes individual detectors 72. Each detector 72 may be tuned, or filtered, to sense light within a predetermined spectral region.
As shown, slit 74 is designed to be of a variable size in the vertical dimension, generally designated as 75. Accordingly, if the dispersed light at the edge of spectrum 78 is ultraviolet (UV) and the middle of the spectrum is in the visible range, then the spectrum becomes uniform in intensity upon reaching detectors 72. Uniform intensity is achieved by the variable sized slit, as the visible light in the middle region of a narrow slit is much more attenuated than the ultraviolet light at an end region of a wider slit.
The dispersive spectrometer system shown in FIG. 7 is described in more detail in U.S. Pat. No. 5,784,158, issued to Stanco, et al., on Jul. 21, 1998, and is incorporated herein by reference in its entirety.
Conventional dispersive spectrometers assume that the scene radiance does not vary while the radiance spectrum is being measured. If the scene radiance does in fact vary—for example, because the line of sight (LOS) of the instrument changes for an inhomogeneous scene—then the radiance spectrum may not be accurately measured. If the LOS change is random, this may become a significant contribution to the random measurement error. In meteorological sounding applications using dispersive spectrometers, the scene inhomogeneity caused by the presence of clouds is a significant source of noise or uncertainty when trying to determine the cloud-free radiances. Conventional retrieval algorithms attempt to remove inhomogeneity using methods such as “cloud clearing,” “hole hunting,” and various other cloud-masking techniques. None of these methods, however, can unambiguously determine the degree of scene inhomogeneity for an individual field-of-view (FOV), because they require observation of multiple or adjacent fields of view.
Determining the degree of scene inhomogeneity for each observed instrument FOV, independently of cloud-clearing or other method, could significantly improve the accuracy of the sounding algorithms used to retrieve atmospheric parameters of interest. Knowing that the FOV contains a homogeneous scene would also be advantageous, because it would permit neglecting the effect of small, uncontrollable, and random LOS changes on the measured spectra.